My three major interrelated objectives are geared towards providing a better understanding of the phylogeny and ontogeny of immunocompetent cells. My first objective is to utilize transplantation of chromosomally tagged lymphoid cells and organs to define the sources of stem cells and migration pathways of frog lymphocytes during embryonic, larval, and adult life. My second objective is to trace the evolution of thymus-dependent and thymus-independent immune responsiveness by defining various immunologic potentialities of lymphocytes from intact and thymectomized amphibians. Those hallmarks of mammalian lymphocytes which will be sought for with lymphocytes from selected Amphibia include the ability to recognize antigen by membrane-associated immunoglobulin receptor sites, the ability to respond to specific and nonspecific mitogens by blast cell transformation and mitosis, and the ability to elaborate soluble mediators that affect the behavior of other cells. My third objective is to gain insight into the phylogeny of histocompatibility systems by studying in mixed lymphocyte culture, the responses of lymphocytes from different amphibians which reject skin allografts with varying degrees of chronicity. I also propose to study the evolution of the complex immunodestructive and immunoprotective (blocking) responses to histocompatibility antigens of varying immunogenicities in the Amphibia.